Exchange interaction in argentum iodide nanocrystals

The results on the selective photoluminescence (PL), photoluminescence excitation (PLE) and the degree of linear polarization (DLP) for the PL spectrum in AgI nanocrystals (NC) embedded in a glass host matrix are presented. The measurements indicate a fine structure of the exciton ground state due to the electron–hole exchange interaction: the PL originates from the lowest triplet state (“dark” exciton) when the higher singlet state (“bright” exciton) is excited. The obtained negative DLP confirms that the emitting state is not directly excited by the pump light, but is created through the virtual interaction with a higher excitonic state of different polarization features and subsequent relaxation. The corresponding red shift of the PL spectrum with respect to the PLE spectrum has been studied in dependence on the NC size. The exchange splitting shows a pronounced size dependence strongly increasing with a restriction of the dot radii. The value of the exchange energy of the exciton for AgI bulk crystals is determined. Both the short- and long-range contributions to the exchange splitting are discussed.